This is program six of video tells series on practical marine electrical
knowledge the series is made up of eight programs program six deals with special
electrical practice relating to vessels designed to carry flammable cargo it
also aims to develop a greater awareness of the safety elements and design
features built into the electrical system of such ships we shall also show
the special electrical equipment certified for use in hazardous areas
demonstrate the servicing of a flameproof light fitting and the method
of fitting a cable gland to a flameproof motor
there are many system variations around so it is most important that you become
familiar with the components of the electrical system and the layout of the
main switchboard immediately you join a ship pay particular attention to the
layout of the emergency switchboard this study will pay dividends during a
blackout or when troubleshooting the cause of a major breakdown
now we must emphasize electrical safety the golden rule is before any work is
done on an electrical installation first isolate the circuit by removing the
supply fuses or locking the circuit breaker in the open position so that the
circuit cannot be energized accidentally then post a warning sign to alert others
that the circuit is being worked on then prove the circuit dent with a
voltmeter or an approved line tester a switchboard can never be considered dead
unless all AC generators connected to it are stopped locked off and all other
supplies are disconnected these points can never be emphasized strongly enough
the electrical rescue procedure is described in program one of this series
a variety of instrumentation on board ship guards controls or monitors the
ship's functioning the red shaded areas on this drawing show the hazardous areas
relating to the Electrical Code of practice which is the subject of this
program generally they are cough next two cargo tanks compressor rooms
and pump rooms and about a three meter periphery surrounding all outlets for
the flammable cargo as far as the electrical system is concerned the
problems here relate to the cables going through these areas and the servicing of
electrical plant operating therein also they involve the use of intrinsically
safe electrical instrumentation and equipment as a general recommendation
hull returns and neutral earth systems are not normally allowed except in the
case of some high voltage systems of 3.3 kV and above where the circuits are not
allowed in any dangerous area also the generating plant switchboards and
battery systems must be separated from the cargo tanks by a gas tight cofferdam
cables laid along the four decks should be installed in steel piping in such a
way as to allow for any structural movement of the ship without strain or
chafing these rectangular expansion boxes house a loop in the cable runs to
allow for expansion the piping to and from the boxes also allows for the
expansion movement of the ship through runs of cables may be permitted through
pump rooms copper dams or enclosures next to cargo
tanks provided they run in heavy gauge steel piping with gas tight joints
cables which are associated with intrinsically safe equipment must be
separated from all other cables and clearly identified as such here the blue
color code is used they will have their own separate power supplies and a zener
protection barrier the Zener barrier ensures that the
maximum preset voltage in the circuitry cannot rise above the prescribed safety
level which ensures the certification of intrinsically safe
EXI EXI or intrinsically safe equipment and circuitry has a protection technique
which confines the energy within the system below a level which is capable of
causing an explosion in a dangerous atmosphere exd certified equipment is
flame proof and relies on the integrity of its encasement to withstand any
internal pressure of an explosion without any flame being emitted exp type
equipment on the other hand is equipment which is protected by a pressurized
inner atmosphere made up of air or inert gas this prevents the possibility of a
dangerous air vapor mixture entering the enclosure of the electrical apparatus
examples of these are seen here
light fittings in hazardous areas must be a flameproof
or pressurized types these light units must be on at least two independent
final branch circuits so that one may be disconnected for maintenance while the
other supplies enough light for safe access switches for these lights must be
double pole interrupting both supply lines and must be situated outside the
hazardous area the maintenance of a flameproof light fitting involves two
specific objectives first the prevention of any deterioration in the equipment
safety level and second ensuring it's correct
safe function
use only naam abrasive materials for cleaning lightly grease then wipe clean
the flanges before fitting them together
periodic maintenance inspection and cleaning will necessitate the inspection
of all flanges glassware and cementing to ensure that they are within the
certified safety level if either the cementing or the glass envelope appear
damaged do not try to mend or replace either the whole fitting has to be
replaced by a new part obtained from the manufacturer it's important to remember
that there is a great deal of difference between flameproof and weatherproof
fittings do not put a gasket between the flanges of a flameproof fitting to make
it weatherproof this will invalidate its flameproof certificate as will any other
modification of the original design
when reassembling these types of fittings extra care must be taken to
ensure that the flanges are clean
the special flameproof cable gland must be checked for tightness and the
electrical connection made again ensuring that the conductor is held
tightly in the terminal box there must be no loose wire strand left in the
terminal box solder the strands of a multi strand conductor to eliminate the
possibility of short-circuit and ensure a secure connection
when tightening any bolts or nuts holding flanges together the force used
should not be more than necessary to prevent loosening by vibration
excessive force may cause undetected fracture of a boat a casting or other
parts of the securing mechanism if a broken stud has to be removed from a
blind hole great care must be taken to avoid damaging the internal thread or
drilling through into the flameproof enclosure
broken or missing screws must be replaced by ones of the same diameter
thread length type and quality of steel
finally check once again that the flameproof gland is correctly tightened
remember the installation must not be changed in any way for any reason as
this may invalidate its certified safety level electrical motors which drive
cargo pumps or compressors are housed in the motor room and they're separated
from the compressor room by a gas tight bulkhead the drive shaft has to be
fitted with gas tight glands where it passes through the bulkhead
the motors are required to be the pressurized type or increased safety Exe
construction
the starters and the control gear for these motors must be housed elsewhere
outside the hazardous area the motor room is pressurized so that gas or vapor
may not enter the air space where the motors are operating entrance to the
motor room is through a system of double doors controlled by a pair of micro
switches and the safety light indicator the system should ensure that one door
is opened only when the other is shut
when a cable run is laid to a flameproof motor special attention must be given to
the preparation of the cable end and fitting the special flameproof gland
onto the cable
here the required length of the outer sheath is stripped off to reveal the
steel armor II allow plenty of time for this work and use sharp tools
the armoring is then cut round with a wire cutter and pulled off
a further short piece of the outer sheath is stripped back as you see here
this will free a further length of armoring which will be clamped between
the double rings of the gland to put tight and secure fitting use special
flameproof glands which have longer threaded flame paths and rubber seal
rings with extended hold on the cable forming another flame cloth carefully
dismantled the cable gland and secure the threaded base unit in the terminal
box
fit the lock nuts on the cable
then the double rings for gripping the steel armory
fit the conical rings below and above the loose armoring like this ready for
tightening
temporarily remove the rubber seal ring from the terminal box port and feed
through the cable
tightening the two pressure fittings with a spanner
this would ensure that the two conical rings gripped the armoring tightly
remove the cable from the terminal box
and replace the rubber seal ring
prepare the conductor ends for receiving the terminal lungs and feed through the
cable again into the terminal box
now the conductor ends are fitted into crimped lugs
and the cable connected to the terminals
check that the lugs hold the cable tight so that they will not vibrate loose when
the motor is operating
nuts are tightened sufficiently to avoid becoming loose due to vibration finally
check their labels to ensure that all portable apparatus such as portable
radios torches or insulation resistance testers are specially approved
intrinsically safe types certified for use in hazardous areas if you use
portable electrical equipment or tools ensure that their connections and cables
are sound and safe to use in the area in which you intend to use them this
concludes the subject for program six here we dealt with the special
electrical safety design features which are built into a vessel designed to
carry hazardous cargo we have dealt with the essential difference between general
electrical practice and specific electrical practice in hazardous areas
we have seen how to service a flameproof light fitting and how to prepare the
cable and cable gland for a flameproof motor emphasis was laid on maintaining
the equipment's certified safety level at all times this is the most important
single difference between working on equipment for use in hazardous areas and
other general electrical installations and lastly we dealt with the use of
intrinsically safe portable equipment we recommend that you watch this program
again and that you consult the book practical Marine electrical knowledge
which accompanies this series and will allow you to study certain aspects in
greater detail finally here's a list of contents for all the programs in the
series